Depends on the quality of your lens. If you have high end gears, you should get the best quality filter. For cheap lens, middle range filters are ok.
I use Hoya HD filters but Pro1D are ok too.
Be careful though, CPL stops part of the light, so if you use it in museums, you’ll need a tripod or put higher ISO setting.

Again photography related, if you do not mind. This time regarding the lenses. Are you using only Tamron 18-270mm one or do you have any additional like 50mm f/1.8 f/1.4 f/1.2 ?
Could you give me some comments wether 50mm large aperture lenses are useful or not? I know they are meant for low-light situations and sharp portraits but has your lens been enough for you so far? Knowing that its max aperture is f/3.5

I use Tamron 18-270 f3.5-6.3, 10-24 f3.5-4.5 and 60mm f2. I also have the Canon 100mm f 2.8 L IS Macro.
Fixed focal lens with wide aperture gives you super sharp pictures with excellent control of focus and depth of field. It can be tricky in some situations when you suddenly need wide angle cause you’ll have to switch lens but whenever time isn’t a problem and when you don’t need a wide focal range, it’s definitely a type of lens that you’ll want to use. If you go for one of those, I recommend getting one that does macro 1:1 (with stabilization if possible : very useful in macro and low light). Since I got my 60 and 100mm, the 18-270 became a secondary lens that I only use when I need a wide zooming range. When taking pictures, going lower than f3.5 isn’t really useful though. The depth of field below f3.5 is really too narrow and you often get part of your subject out of focus. Also these lenses usually perform better around f4.

1)
Right, I know this will sound complete noob-ish but bear with it.
I’m a beginner photographer and my setup is:
Canon EOS 1100D with EF-S 18-55mm f/3.5-5.6 IS II lens.
I am not aiming to become a professional – just taking pictures for myself but I want them to be quite spectacular.
Do you think that lens is good enough for starters or should I get some large aperture/macro/telezoom lens from the very start?
I’m moving to Turkey for half a year there in a month and going to travel a lot around there, so I’m quite a budget person currently – can get an extra lens or two but not something costy (I’m not very into the idea of changing lens since that looks annoying but would have to do that if it’s a must).

2)
About blurry background close photos: which technique is the better:
Max zoom out (barrel distortion but larger aperture) or zoom in (less distortion but smaller aperture + shallower depth of fiend)?

3)
If you remember, what photography guides did you watch on YouTube? (there are a lot there and not all are so professional, obviously) I’m not very into spending watching them all.

1- Your equipment is ok for classic photo but if you want spectacular, I recommend super wide angle. Tamron 10-24 isn’t expensive and gives impressive results. The difference between 18 and 10 might not seem important on paper but in reality it’s night and day. Very useful for building, street photo and landscape. It’s bad for close portrait at 10 but ok at 24. Fixed focal or wide aperture doesn’t do much better than your 18-55 unless you really know what you’re doing, so you can go with it in maybe 6 months~1 year when you get better.
2- With your lens, you’ll get better result at 55mm, depth of field will be a lot easier to control than at 18 where pretty much everything is in focus (even at f5.6). Don’t worry about distortion, it’s easy to correct in post production and unless you’re taking pictures of straight lines, you won’t even notice it.
3- I like to watch DigitalRev TV on YouTube. They review all the new stuff and give useful advise.

Advise from me : never trust your camera screen. A photo will often look perfect on it but when you open it on computer, you’ll notice everything that can be wrong with it. So try to transfer all your photos to computer at least once a day and check if you need to be careful about something that you didn’t pay attention before.